This invention relates to a method of detecting abnormality in an air-fuel ratio control system for internal combustion engines, which employs a proportional control valve controlled in response to an output signal from a sensor for sensing oxygen concentration in exhaust gases, for controlling an amount of secondary air supplied to the engine.
An air-fuel ratio control system for internal combustion engines is already known, e.g. from Japanese Provisional Patent Publication (Kokai) No. 55-119941, which employs a proportional control valve arranged in an air supply passage for supplying secondary air therethrough to the engine and controlled in response to an output signal from an oxygen concentration sensor arranged in the exhaust system of the engine in such a manner that its valve opening varies in proportion to the magnitude of driving current supplied thereto.
The known air-fuel ratio control system is provided with a series circuit comprising a solenoid of the proportional control valve and a driving transistor serially connected between a power source which supplies a predetermined voltage and ground, wherein the valve opening of the proportional control valve, i.e. the amount of secondary air to be supplied to the engine is controlled in proportion to the magnitude of driving current supplied to the solenoid, which current is determined by duty ratio control of the driving transistor.
In the event of a short circuit occurring in the driving transistor or harness connecting between the driving transistor and the solenoid being short-circuited to ground due to biting or a like cause, the solenoid is continuously energized, i.e. energized with almost 100 percent duty ratio, as a result of which the proportional control valve is opened to the maximum degree so that an excessive amount of secondary air is supplied to the engine, resulting in overleaning of the air-fuel ratio of a mixture supplied to the engine and consequently on degraded combustion of the engine.
If such abnormality as a short circuit in the driving transistor is detected by sensing the value of a voltage at a specific location in the series circuit formed of the driving transistor, etc., detection of abnormality cannot be effected accurately while the engine is in a starting condition, since the above voltage sensed is fluctuating due to a drop in the output voltage of a battery feeding power to the starting motor during starting of the engine. This necessitates effecting detection of abnormality after completion of cranking of the engine and thereafter causing the driving transistor for the proportional control valve for secondary air amount control to be energized only when no abnormality has been detected. On the other hand, it is requisite that when the engine is restarted in a hot state, e.g. when it is restarted immediately after it has been stopped, an increased quantity of secondary air should be supplied to the engine for smooth starting of the engine. However, if the driving transistor is faulty, it can burn if it is energized before such faulty condition is detected.